buffers, indicators and pH titration curves Flashcards
define a buffer solution
can resist changes in pH despite the addition of small amounts of acid or alkali
what does a buffer solution contain (2 options)
a weak acid and the salt of that weak acid
or
a weak alkali and the salt of that weak alkali
buffer equilibrium equation example with ethanoic acid and sodium ethanoate
CH3COOH + H2O <> CH3COO- + H3O+
if you add acid to a buffer system what happens
(ethanoic acid and sodium ethanoate)
buffer has a large excess of CH3COOH and CH3COO-
CH3COO- reacts with H+
equilibrium shifts to the left
removing added H+
ratio of [CH3COO-] to [CH3COOH] hardly changes
pH stays roughly constant
if you add alkali to a buffer system what happens
(ethanoic acid and sodium ethanoate)
buffer has a large excess of CH3COOH and CH3COO-
CH3COOH reacts with OH-
equilibrium shifts to the right
removing added OH-
ratio of [CH3COOH] to [CH3COO-] hardly changes
pH says roughly constant
what 2 assumptions do you make when doing a buffer calculation
use ethanoic acid and sodium ethanoate as example
1 [CH3COOH] original = [CH3COOH] equilibrium
2 [CH3COONa] original = [CH3COO-] equilibrium
equation to illustrate dissociation of ethanoic acid
CH3COOH <> CH3COO- + H+
Ka equation (for ethanoic acid and sodium ethanoate)
[CH3COO-] [H+] / [CH3COOH]
steps in buffer calculation (weak acid and it’s salt)
1 buffer equation
2 Ka expression
3 find conc. acid
4 find conc. salt
5 calculate [H+] using Ka equation
6 calculate pH using -log[H+]
steps in buffer calculation (weak acid and strong base)
1 ICEC to find moles of weak acid, strong base and acid salt
2 find conc. acid
3 find conc. salt
4 Ka expression
5 calculate [H+] using Ka equation
6 calculate pH using -log[H+]
how to measure pH
glass electrode
pH metre converts Emf into pH reading
what is the equivalence point
the point at which
moles H+ = moles OH-
what is the significance of the half equivalence point
pH at half equivalence point is equal to the pKa
where does a pH curve start/end with a strong base
pH 12-13
where does a pH curve start/end with a strong acid
pH 1-2
where does a pH curve start/end with weak a base
pH 10-11
where does a pH curve start/end with a weak acid
pH 3-4
pH at equivalence point with salt of strong acid and strong base (why)
7
forms salt and water
pH at equivalence point with salt of a weak acid and a strong base (why)
greater than 7
due to OH- present
pH at equivalence point with salt of a strong acid and a weak base (why)
less than 7
due to H+ present
what is an indicator
a weak acid or a weak base
has one colour in the undissociated form and a different colour as a dissociated ion
what is the characteristic of an ideal indicator for an acid-base titration
one that is in the middle of it’s colour change
at the pH of the equivalence point of the titration
what is the expression for the acid dissociation constant with an indicator (In)
KIn = [H+][In-] / [HIn]
what indicator would you use for weak acid vs strong base and why
phenolphthalein
changes colour completely over vertical section of graph
pKIn value of indicator is approximately equal to equivalence point of titration
what indicator would you use for weak base vs strong acid and why
methyl orange
changes colour completely over vertical section of graph
